1. Field of the Invention
This invention relates generally to the water cooling of rolling mill rolls, and more particularly, to a simple and inexpensive method and apparatus for automatically controlling the cooling rates within various zones of the rolling mill roll or even a hot rolled product exiting a hot roll stand. The invention provides a simple and more reliable control of cooling rates by providing a plurality of nozzles on a spray bar, each providing a continuous and fixed spray of liquid coolant onto the roll or hot rolled product, and automatically adjusts the position of the spray bar with regard to the roll or product being cooled, thereby adjusting the spray-angles, spray-distances, or both, to effect cooling rate adjustments as necessary.
2. Description of the Prior Art
In modern metal rolling mills, there are a variety of differing rolling processes and procedures for producing finished and semi-finished metal products. Typically, heated slabs or billets, (steel or aluminum, for example) produced by continuous casting machines are hot rolled through one or more roll stands to produce finished or semi-finished products, such as plates, structural products, bars, rods, hot strips and the like. Further finishing steps may include cold rolling such as the cold rolling of hot strip to sheet products. Such roll stands generally comprise at least one pair of rolls between which the metal workpiece is passed to reduce and/or shape the metal workpiece as desired.
During the metal rolling operation, mill rolls are continuously heated by a work heat due to the plastic deformation of the rolled metal, a frictional heat generated between the rolled metal and the rolls, and, in the case of hot rolling, heat transfer from hot metal workpiece. Particularly in the case of hot rolling steel where the steel to be rolled is preheated to temperatures in excess of 1200 C., roll heating as a result of heat transfer can become rather excessive.
Because of such roll heating, it is essential in practically all metal rolling operations that means be provided to cool the rolls during use and thereby prevent unwanted thermal expansion of the rolls, which can adversely affect the quality of the rolled product. For example, in the hot rolling of flat rolled products such a plates, strip and sheet, the rolls tend to become excessively heated in their mid-portion in contrast to the edge portions, causing the diameter of the rolls to increase to a greater extent in the mid-portion, and therefore roll a thinned mid-section into the product as compared to the outer sections. In addition, excessively heated rolls will wear more quickly and tend to stick to the rolled metal surface to adversely affect the surface quality of the rolled product.
While numerous differing types of apparatus have been utilized to cool the rolls, most have been based on the provision of a line of coolant spray nozzles spaced along a side surface of the roll parallel to the roll axis, and positioned on either or both the entrance and/or exit side of the roll. Typically, an elongated spray bar; i.e., manifold or header, having a width generally equal to the width of the roll, is closely positioned parallel to the roll, which has a plurality of equally spaced spray nozzles to direct the water or other coolant from the manifold to the rotating roll. It is well known that the cooling rate is not only a function of the amount of coolant sprayed, but also the spray-distance and spray-angle of the coolant sprayed onto the roll. Accordingly, the nozzle distances from the roll and its spray-angles are normally fixed and uniform to provide optimum angle and distance parameters.
While most rolls tend to be uniformly heated circumferentially, they are not normally heated uniformly in the elongated or axial direction, as noted above. Therefore, it is preferred that the coolant nozzles do not uniformly cool the roll across their axial width, but rather achieve a cooling rate in the various circumferential zones of the roll in proportion to the heating rate within the various zones. Specifically, the individual nozzles should be regulated to concentrate the cooling rate at those circumferential areas of the roll which are subjected to higher heating rates (e.g. the center portion of the roll in the case of rolling flat rolled products) so that the overall temperature of the roll surface can be maintained at a reasonably uniform level. Such an effort is essential if nonuniform thermal expansion is to be prevented and proper roll profile maintained to assure proper dimensions and form of the rolled products.
Accordingly, most cooling systems comprise localized (or segmented) systems to effect differing cooling rates within different zones of the rolls. While it is possible to utilize nozzles having different orifice diameters, or provide a varied spacing between the nozzles, the desired cooling rate profile will normally change from time to time, particularly as the rolled product is continually changing its profile and dimensions. The most practical of the prior art systems, therefore, have utilized nozzles having remotely controlled on/off valves so that the cooling rates in the various roll zones can be controlled by selectively turning certain valves on and certain valves off. Typically, the coolant manifold or spray bar is divided into multiple segments, with each segment containing several nozzles. By selecting an appropriate number of properly positioned nozzles to be turned on, a proper coolant flow pattern can be selected to achieve a suitable cooling rate for each zone. Some such systems utilize a closed-loop control which can turn valves on and off in response to a need to change the cooling rate in any one or more particular segments.
While such cooling systems are generally satisfactory, they do leave a lot to be desired. The most notable problem being the fact that the on/off valves are rather intricate and do not always function properly in the harsh hot rolling mill environment. If a valve remains off or on for a considerable period of time, the heat in the vicinity may at times cause it to "freeze" in that off or on position, or process debris may plug a closed nozzle so that it cannot thereafter be reopened. Accordingly, the reliability of the valved nozzles is quite unsatisfactory, and leads to either considerable down-time to repair or replace one or more nozzles, or less than optimum cooling rate control of the rolls.
Another short-coming of the prior art systems is that since the manifolds and nozzles are fixed, the spray-distances and spray-angles are fixed, as noted above. If only one set of rolls is ever utilized in a particular roll stand, there is no particular problem. With regard to many roll stands, however, it is common practice to change the rolls from time to time for purposes of rolling different products which requires exchanging one set of rolls for a set of rolls of a different diameter. Therefore, since the spray-distances and spray-angles are fixed at optimum parameters for one given set of rolls, they will not be at optimum positions when rolls of a different diameter are substituted.